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easyBasicsSometimes asked

Convert to Lowercase

Basics · 6 min

Given a string s, return it with every uppercase letter converted to lowercase. Non-letters are unchanged.

In plain English

Mirror of to-uppercase: add 32 to uppercase letters.

Examples

Example 1

Input: s = "HELLO"
Output: "hello"
Note: All lower-cased.

Example 2

Input: s = "AbC1"
Output: "abc1"
Note: Digit unchanged.

Example 3

Input: s = "z"
Output: "z"
Note: Already lower.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: ASCII only?Explore questions

Q1.ASCII only?
Yes for the manual shift; the library covers Unicode/locale.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

For each char in 'A'..'Z', add 32 to its ASCII code; leave others unchanged. Or call `toLowerCase()`.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you apply the case shift in the correct direction with a guard.

Where candidates slip

Wrong sign, or shifting non-letters.

How to stand out

Mention locale-aware lowercasing for non-ASCII.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

ASCII shift

TimeO(n)SpaceO(n)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

Same 32 gap — which direction lowercases?

2Hint 2Explore hint

Add 32 to A–Z letters; guard the range.

3Hint 3Explore hint

Leave non-letters untouched.

A · Core idea

The mirror of to-uppercase: add 32 to an uppercase letter to get its lowercase form, since 'A'=65 and 'a'=97 differ by exactly 32.

Why guard A–Z. Only uppercase letters should change; the range check leaves digits, spaces, and symbols alone. Adding 32 to a non-letter would produce garbage.

Direction. Add 32 to go down to lowercase; subtracting would uppercase. Keeping the two directions straight (−32 up, +32 down) is the whole trick.

Library note. toLowerCase() handles locale and non-Latin scripts; the manual shift is the ASCII demonstration.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

s = "Ab1"

in A-Z → +32

Statea

Final result"ab1"

C · Code with teaching comments

solution.javaJava

ASCII shift — optimal

1
public String toLower(String s) {
2
    char[] c = s.toCharArray();
3
    for (int i = 0; i < c.length; i++) {
4
        if (c[i] >= 'A' && c[i] <= 'Z') c[i] += 32;
5
    }
6
    return new String(c);
7
}

D · Why it works and complexityO(n) timeO(n) spaceOpen the invariant and proofExplore the proof

ComplexityO(n) time, O(n) space — one range check and possible shift per character.

E · Edge cases and pitfalls5s = "HELLO" — → "hello".Explore edge cases

Edge cases handled

s = "HELLO"→ "hello".
s = "AbC1"Digit unchanged → "abc1".
s = "z"Already lower → "z".

Pitfalls

Subtracting 32 (uppercases) by mistake.
Touching non-letters.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic String toLower(String s) { char[] c = s.toCharArray(); for (int i = 0; i < c.length; i++) { if (c[i] >= 'A' && c[i] <= 'Z') c[i] += 32; }Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Subtracting 32 (that uppercases) by mistake.
Touching non-letters.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

ASCII case shift

Code pattern to memorize

if 'A'<=ch<='Z': ch += 32

Rules to remember

1Lower = upper + 32.
2Guard with A-Z range.
3Leave others alone.

Use this pattern when

ASCII case manipulation

Interview takeaway

Lowercasing is +32; uppercasing is −32.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: ASCII case shift. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Convert to Uppercase
Subtract 32 instead.

easyBasicsSometimes asked

Convert to Lowercase

Basics · 6 min

Given a string s, return it with every uppercase letter converted to lowercase. Non-letters are unchanged.

In plain English

Mirror of to-uppercase: add 32 to uppercase letters.

Examples

Example 1

Input: s = "HELLO"
Output: "hello"
Note: All lower-cased.

Example 2

Input: s = "AbC1"
Output: "abc1"
Note: Digit unchanged.

Example 3

Input: s = "z"
Output: "z"
Note: Already lower.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: ASCII only?Explore questions

Q1.ASCII only?
Yes for the manual shift; the library covers Unicode/locale.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

For each char in 'A'..'Z', add 32 to its ASCII code; leave others unchanged. Or call `toLowerCase()`.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you apply the case shift in the correct direction with a guard.

Where candidates slip

Wrong sign, or shifting non-letters.

How to stand out

Mention locale-aware lowercasing for non-ASCII.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

ASCII shift

TimeO(n)SpaceO(n)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

Same 32 gap — which direction lowercases?

2Hint 2Explore hint

Add 32 to A–Z letters; guard the range.

3Hint 3Explore hint

Leave non-letters untouched.

A · Core idea

The mirror of to-uppercase: add 32 to an uppercase letter to get its lowercase form, since 'A'=65 and 'a'=97 differ by exactly 32.

Why guard A–Z. Only uppercase letters should change; the range check leaves digits, spaces, and symbols alone. Adding 32 to a non-letter would produce garbage.

Direction. Add 32 to go down to lowercase; subtracting would uppercase. Keeping the two directions straight (−32 up, +32 down) is the whole trick.

Library note. toLowerCase() handles locale and non-Latin scripts; the manual shift is the ASCII demonstration.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

s = "Ab1"

in A-Z → +32

Statea

Final result"ab1"

C · Code with teaching comments

solution.javaJava

ASCII shift — optimal

1
public String toLower(String s) {
2
    char[] c = s.toCharArray();
3
    for (int i = 0; i < c.length; i++) {
4
        if (c[i] >= 'A' && c[i] <= 'Z') c[i] += 32;
5
    }
6
    return new String(c);
7
}

D · Why it works and complexityO(n) timeO(n) spaceOpen the invariant and proofExplore the proof

ComplexityO(n) time, O(n) space — one range check and possible shift per character.

E · Edge cases and pitfalls5s = "HELLO" — → "hello".Explore edge cases

Edge cases handled

s = "HELLO"→ "hello".
s = "AbC1"Digit unchanged → "abc1".
s = "z"Already lower → "z".

Pitfalls

Subtracting 32 (uppercases) by mistake.
Touching non-letters.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic String toLower(String s) { char[] c = s.toCharArray(); for (int i = 0; i < c.length; i++) { if (c[i] >= 'A' && c[i] <= 'Z') c[i] += 32; }Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Subtracting 32 (that uppercases) by mistake.
Touching non-letters.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

ASCII case shift

Code pattern to memorize

if 'A'<=ch<='Z': ch += 32

Rules to remember

1Lower = upper + 32.
2Guard with A-Z range.
3Leave others alone.

Use this pattern when

ASCII case manipulation

Interview takeaway

Lowercasing is +32; uppercasing is −32.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: ASCII case shift. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Convert to Uppercase
Subtract 32 instead.